To provide focused vision, an eye must be capable of focusing light on the retina. An eye's ability to focus light on the retina depends, to a large extent, on the shape of the eyeball. If an eyeball is “too long” relative to its “on-axis” focal length (meaning, the focal length along the optical axis of the eye), or if the outside surface (that is, the cornea) of the eye is too curved, the eye will be unable to properly focus distant objects on the retina. Similarly, an eyeball that is “too short” relative to its on-axis focal length, or that has an outside surface which is too flat, will be unable to properly focus near objects on the retina.
An eye that focuses distant objects in front of the retina is referred to as a myopic eye. The resultant condition is referred to as myopia, and is usually correctable with appropriate single-vision lenses. When fitted to a wearer, conventional single-vision lenses correct myopia associated with central vision. Meaning that, conventional single-vision lenses correct myopia associated with vision that uses the fovea and parafovea. Central vision is often referred to as foveal vision.
Although conventional single-vision lenses can correct myopia associated with central vision, recent research has shown (reviewed in R. A. Stone & D. L. Flitcroft (2004) Ocular Shape and Myopia, published in Annals Academy of Medicine, Vol. 33, No. 1, pp.7-15) that off-axis focal length properties of the eye often differ from the axial and paraxial focal lengths. In particular, myopic eyes tend to display less myopia in the retina's peripheral region as compared with its foveal region. This difference may be due to a myopic eye having a prolate vitreous chamber shape.
Indeed, a recent United States study (Mutti, D. O., Sholtz, R. I., Friedman, N. E., Zadnik, K. Peripheral refraction and ocular shape in children, Invest. Opthalmol. Vis. Sci. 2000; Vol. 41, pp. 1022-1030) observed that the mean (±standard deviation) relative peripheral refractions in myopic eyes of children produced +0.80±1.29 D of spherical equivalent.
Interestingly, studies with chicks and monkeys have indicated that a defocus in peripheral retina alone, with the fovea staying clear, can cause an elongation of the foveal region (Josh Wallman and Earl Smith independent reports to 10th International Myopia Conference, Cambridge, UK, 2004) and the consequent myopia.
On the other hand, epidemiological studies have shown the presence of correlation between myopia and near work. It is well known that the prevalence of myopia in the well educated population is considerably higher than that for unskilled labourers. Prolonged reading has been suspected of causing a hyperopic foveal blur due to insufficient accommodation. This has led many eye care professionals to prescribing progressive addition or bi-focal lenses for juveniles manifesting progression of myopia. Special progressive lenses have been designed for use by children (U.S. Pat. No. 6,343,861). The therapeutic benefit of these lenses in clinical trials has been shown to be statistically significant in retarding progression of myopia but clinical significance appears to be limited (e.g., Gwiazda et al., 2003, Invest. Opthalmol. Vis. Sci., Vol. 44, pp. 1492-1500). However, Walker and Mutti (2002), Optom. Vis. Sci., Vol. 79, pp. 424-430, have found that accommodation also increases the relative peripheral refractive error, possibly due to the increased choroidal tension during accommodation pulling the peripheral retina inward.
Unfortunately, conventional myopia correcting lenses haphazardly produce clear or defocused images in the retina's peripheral region. Thus, existing ophthalmic lenses for correcting myopia may fail to remove stimuli for myopia progression.
The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of any of the claims.